Sea Water Reverse Osmosis (SWRO) Desalination Plant ...
Transcript of Sea Water Reverse Osmosis (SWRO) Desalination Plant ...
Sea Water Reverse Osmosis (SWRO)
Desalination Plant Concept and Conceptual Design
800,000 cubic meters per day
February 2019 PESCO
2
Table of Content
1.0 Introduction
2.0 Production of Fresh Water Through Desalination
3.0 Reverse Osmosis (RO) System 3.1 RO System Component 3.2 Membrane Configuration and Material 3.3 Energy Recovery in RO System 3.4 Operation and Maintenance of RO System
4.0 RO Desalination Economic Evaluation 4.1 System Configuration 4.2 Water Cost 4.3 Sea Water RO System Cost 4.4 Electric Price Impact on Water Cost 4.5 Water Cost Reduction Factors
Appendix A:
Conceptual Design for 800,000 cubic meters per day SWRO Plant.
3
Proposed Concept for Reverse Osmosis (RO)
Desalination Plants
1. Introduction
As most countries globally moved forward in its economic reform program to a more
market-based economy, Egypt has joined the other countries that are committed to
private sector participations in their infrastructure projects. With the increase in
electricity demand to support mainly the industrial and domestic use, the Egyptian
government has shifted its resources to welfare projects and emphasized on the
participation of the private sector in the electric power generation projects.
Accordingly, Egypt has achieved its goal for electric power generation installed capacity.
Currently, Egypt possess over than 10,000 MW of excess capacity ready to be utilized for
generating the needed fresh water requirements through available and appropriate
desalination technologies.
2. Production of Fresh Water Through Desalination
Desalination is a separation process used to reduce the dissolved salt content of saline
water to a usable level. The earliest form of desalination was accomplished by boiling the
salt water, then cooling and condensing as fresh water. The best-known thermal
technologies are the following:
Multi-Stage Flash (MSF), Multi-Effect Distillation (MED), and Vapor Compression (VC).
The newest commercial technology for desalination is based on membrane treatment.
Brackish Water Reverse Osmosis (BWRO), or Sea Water Reverse Osmosis (SWRO), is the
fastest growing desalination technique with the greatest number of installations around
the globe; it is beginning to dominate the current and future desalination markets. Its
energy consumption is usually some 70% less than for comparable evaporation
technologies.
Advancements have been made in membrane technology, resulting in stable, long-lived
membrane elements. Component parts have been improved, as well, reducing
maintenance and down time. Additional advancements in pretreatment have been made
4
in recent years, further extending membrane life and improving performance. Reverse
osmosis delivers product water or permeate having essentially the same temperature as
the raw water source (an increase of 1C may occur due to pumping and friction in the
piping). This is more desirable than the hot water produced by evaporation technologies.
RO systems can be designed to deliver virtually any required product water quality. For
these and other reasons, RO is usually the preferred method of desalination today.
A disadvantage of RO is the need for significant pre-conditioning of the feed water to
protect the membranes. The extent of pre-treatment requirements depends on a variety
of factors, such as seawater composition and temperature, seawater intake, membrane
materials, and recovery ratio.
Typical electricity consumption of SWRO plants is in the range of 3 to 4 kWh/m3,
depending on sea water salinity, recovery ratio, required permeate quality, plant
configuration, and energy recovery in the brine blow down.
3. Reverse Osmosis (RO) system
Reverse osmosis is a membrane separation process in which pure water passes from the
high- pressure seawater side of a semipermeable membrane to the low-pressure
permeate side of the membrane. To overcome the natural osmotic process, the seawater
side of the system has to be pressurized to create a sufficiently high net driving pressure
across the membrane. In practice, the seawater can be pressurized to pressures as high
as 70 to 80 bars. The remaining feed water continues through the pressurized side of the
unit as brine. No heating or phase change takes place.
3.1 RO System Component
The two most basic individual components in a seawater RO system are the high-
pressure feed pump and the RO membranes. These components comprise the heart
of any RO system and require careful selection and application for successful
operation. In addition to these, other components related to the pretreatment of
the inlet water and adjustment of the product water are also included.
Pretreatment. The incoming feed water is pretreated to be compatible with the
membranes by removing suspended solids, adjusting the pH, and adding a threshold
inhibitor to control scaling caused by constituents such as calcium sulfate.
Pressurization. The pump raises the pressure of the pretreated feed water to an
operating pressure appropriate for the membrane and the salinity of the feed water.
5
Separation. The permeable membranes inhibit the passage of dissolved salts while
permitting the desalinated product water to pass through. The saline feed is pumped
into a closed vessel where it is pressurized against the membrane. As a portion of
the water passes through the membrane, the salt content in the remaining brine
increases. Portion of this brine is discharged without passing through the
membrane.
Stabilization. The product water from the membrane assembly usually requires pH
adjustment and degasification before being transferred to the distribution system
for use as drinking water. The product passes through an aeration column in which
the pH is elevated from a value of about 5 to close to 7.
Schematic diagram of an RO system.
3.2 Membrane Configuration and Material
RO membranes come in a variety of configurations. Two of the commercially successful configurations are the spiral-wound module and hollow-fiber module. In both configurations, module elements are serially connected in pressure vessels (up to seven in spiral-wound modules and up to two in hollow-fiber modules).
Spiral – Wound module A spiral-wound module element consists of two membrane sheets supported by a grooved or porous support sheet. The support sheet provides the pressure support for the membrane sheets, as well as providing the flow path for the product water. Each sheet is sealed along three of its edges, and the fourth edge is attached to a central product discharge tube. A plastic spacer sheet is located on each side of the membrane assembly sheets, and the spacer sheets provide the flow channels for the feed flow. The entire assembly is then spirally wrapped around the central discharge tube forming a compact RO module element. The recovery ratio (permeate flow rate divided by the feed flow rate) of spiral-wound module elements is very low, so up to seven elements are arranged in one module to get a higher overall recovery rate.
6
Spiral-wound RO module element.
Hollow – Fiber Module
Hollow-fiber membranes are made of hair-like fibers, which are united in bundles and arranged in pressure vessels. Typical configurations of hollow-fiber modules are U tube bundles, similar to shell and tube heat exchangers. The feed is introduced along a central tube and flows radially outward on the outside of the fibers. The pure water permeates the fiber membranes and flows axially along the inside of the fibers to a “header” at the end of the bundle.
Hollow fiber module
3.3 Energy Recovery in RO System
A key criterion for the RO system is the specific electricity consumption, which should be as low as possible. That means that the recovery ratio must be kept as high as possible and the accompanying feed water pressure as low as possible, fulfilling the drinking water standards as well as the design guidelines of the manufactures. Because the overall recovery ratios of current seawater RO plants are only 30% to 50%, and because the pressure of the discharge brine is only slightly less than the feed stream pressure, all large-scale seawater RO plants, are equipped with energy-recovery turbines that recover a part of the pumping energy.
7
Recent advances in energy-recovery device technology, together with improved membrane technology and process operations, have reduced the energy required by SWRO to a level comparable to the energy required to pump and treat surface water in many locations. A number of turbine-based centrifugal energy recovery devices such as the Pelton wheel, Francis, and Reversal pump have been employed since the 1980s to recover pressure energy from the membrane reject stream and return it to the feed of the RO process.
RO unit with a Pelton turbine energy recovery device
3.4 Operation and Maintenance of RO system
Assuming that a properly designed and constructed RO unit is installed, the major operational elements associated with the use of this technology will be the day-to-day monitoring of the system and a systematic program of preventive maintenance. Operation, maintenance, and monitoring of RO plants require trained engineering staff. Staffing levels are about one person for a 200 m3/day plant, increasing to three persons for a 4,000 m3/day plant. Preventive maintenance includes instrument calibration, pump adjustment, chemical-feed inspection and adjustment, leak detection and repair, and structural repair of the system on a planned schedule. The main operational concern related to the use of RO units is fouling, caused when membrane pores are clogged by salts or obstructed by suspended particulates. It limits the amount of water that can be treated before cleaning is required. Membrane fouling can be corrected by backwashing or cleaning, and by replacement of the cartridge filter elements.
4. RO Desalination Economic Evaluation
4.1 System configuration
The selection of process components of RO system is affected to the great extent by the type of water the membrane plant will process. In general a RO plant will consist of the following system components and treatment steps:
8
A. Raw water source • Sea Water Intake
B. Pretreatment
• Screening Settling • Coagulation • Filtration (conventional or membranes – media filtration) • Chemical conditioning (acid and/or scale inhibitor) • Cartridge filtration
C. High pressure pumping unit
• High pressure pumps • Power recovery equipment
D. RO trains
• Permeate treatment, conditioning and storage. • Instrumentation and control • Electric system including motor control center • Membrane cleaning unit.
4.2 Water Cost
In the last decade there was a significant decrease of capital and operating cost. Desalted water cost, supplied to customer. The drivers behind the economical improvements are competition and improvement of process and membrane technology. A majority of large RO systems are built to provide water to municipalities, usually in the framework of build, own and operate (BOO) arrangements. The desalination projects are awarded as result of a very competitive bidding process. Competitive bidding process affected prices of every equipment component of RO systems (including membrane elements) and resulted in a broad price decline. Better performance of equipment and optimization of process design resulted in lower operating cost. The water cost is composed of capital cost, power consumption, maintenance and parts, membrane replacement, consumables and labor. The major cost components in seawater RO systems are power and capital cost.
4.3 Seawater RO System Cost
The RO system cost is calculated through cost contribution of major system components:
9
• Site preparation and building • Intake and outfall • Pretreatment • RO trains • RO membrane elements • Piping • High pressure pumps and power recovery turbines • Electrical • Permeate post-treatment and storage • Membrane cleaning system • Instrumentation and control system • Contingency • Engineering • Owners cost • Interest during construction
The construction cost of large capacity RO seawater desalination plants is currently reported to be approximately $800 /m3-day. The following are the major assumptions used to calculate the net water cost for large RO plant based on BOO concept: Plant Size 800,000 m3/day Capital Cost $800/m3-day Discount Rate 6% Plant life 25 years
It should be noted that, the plant construction cost is a location specific, depending among other issues on the length of the project preparation process and process requirements in respect of raw water quality and product water quality specifications. The summary of individual cost components for large RO plant (800,000 m3/day) is as follows:
Production Water Cost Component $/m3
a. Capital cost (Capital Recovery, 12 years 0.290
@6% Discount Rate)
b. Electric Power ( $0.07/kwh) 0.210
c. RO membrane replacement (6 years member life) 0.052
d. Media Filter membrane replacement 0.039
(7 years membrane life)
e. Chemicals 0.030
f. Maintenance and spare parts 0.036
g. Labor 0.028
Total cost 0.685
Note: An increase in the duration of the Capital Recovery from 12 to 25 years set
at $0.1713/m3, will result in water production cost to be approximately $0.567/m3.
10
It should be noted that the cost associated to the connection to water distribution
system should be added to the above water production cost. This will be subject to
the project site and the target distribution zone.
4.4 Electric Price Impact on Water Cost
Because the high-pressure pump represents the heart of the RO unit, the cost of electricity consumed by the pump and other electricity consuming devices in the RO plant should have a significant effect on the water production cost. We assume in this analysis that the electricity is supplied from the grid or another central source in the region. The analysis shows that increasing the electricity price from US$0.06 to US$0.08 per kWh will increase the cost of water by about 15%.
4.5 Water Cost Reduction Factors
An increase in recovery rate and permeate flux (rate of fresh water passing through RO membrane) in seawater systems can improve the economics of the desalting process. Implementation of high recovery (permeate rate to total feed water flow), high flux operation requires better quality of the feed water. New capillary membrane technology used as a pretreatment step has the potential to produce feed water quality which will enable to operate seawater membranes at a higher flux rate. The new technology has demonstrated reliable operation at variety of operating conditions. The associated conceptual design for the proposed 800,000 cubic meters per day SWRO plant is exhibited in Appendix A.
Appendix A
Conceptual Design
For
800,000 Cubic meters per day SWRO Desalination
Plant
1
Table of Contents
1.0 Introduction: .................................................................................................................... 2
2.0 Process Description: ......................................................................................................... 3
3.0 Desalination Plant Design Bases: .................................................................................... 5
4.0 Intake Water System: .................................................................................................... 10
5.0 Brine Water Discharge: ................................................................................................. 12
6.0 Pre-treatment of Sea Water: ......................................................................................... 13
7.0 Desalination RO System: ............................................................................................... 14
8.0 Degasification and Re-mineralization: ......................................................................... 14
8.1 Degasification ................................................................................................................ 14
8.2 Re-mineralization .......................................................................................................... 14
9.0 Conclusion:..................................................................................................................... 15
Attachments:
1. Process Flow Diagram
2. Intake Data & Design guidelines
3. Equipment Sizing
4. Preliminary Auxiliary Equipment Electrical Load
5. Site Selection Criteria
2
1.0 Introduction:
Water scarcity had been serious concern amid scientists, politicians and business
communities over last several decades. Available surface or fresh water (river, lakes and
ground water) isn't enough for ever growing world population. Climate change and urban
growth continue to widen the shortfall of drinking water availability globally. A sustainable
solution to meet water demand of this century has to come from sea water.
Egypt has long experience of operating desalination plants built with both thermal and
membrane technologies. Over last two decades, considerable performance improvement of
membrane technologies made Sea Water Reverse Osmosis (SWRO) desalination plant
commercially viable method of producing drinking water at a lower cost. Approximately, 40
desalination plants in Egypt produce desalinated water to meet demand for drinking water.
Egypt's current development plan looks for additional SWRO plants to meet future demands
of drinking water through 2050.
A business proposal for 800,000 m3/day SWRO Desalination plant under "Build, Operate and
Own (BOO)" or "Build, Operate, Own and Transfer (BOOT)" schemes well align with Egypt’s
current economic plans. Currently, Egypt has surplus electric power generation. The driving
economic consideration is to utilize this excess power reserve to operate large scale SWRO
desalination projects.
The desalination plant is very much site specific in terms of water quality, design of the Intake
system, selection of pre-treatment water equipment and selection of RO systems equipment.
Since no specific site is selected at this time, the proposed design remains conceptual.
A conceptual Flow Diagram in Attachment 1 presents the system configuration, process
flows, equipment and associated piping. It also includes important process parameters from
water intake to product water output to the Municipality network distribution.
The type and configuration of sea water intake has significant impact on nature and quantity
of foulants that needs to be considered in selection of pre-treatment equipment and RO
membranes. This conceptual design is based on an open intake system, typically used in large
desalination plants. Other option is subsurface saline water intake. This method has
qualitative advantages in terms of low salinity, TSS and TDS concentrations. Therefore, sub-
surface sea water intake should also be reviewed for site selection.
3
2.0 Process Description:
The Flow Diagram exhibited in Attachment 1 shows the details of the entire desalination plant
process. A 3-D perspective of a typical Membrane filtration and RO Module is presented in
Figure-1 below for a quick understanding of the proposed SWRO Desalination plant. The
design includes state-of-the-art membrane filtration technology to pre-treat the source water
and final reverse osmosis (RO) modules to produce clean drinking water. Membrane
technology consisting of bank of micro-filters (MF) and Ultra-filters (UF) will be used to
remove suspended solids (TSS), viruses and bacteria of sizes greater 0.01 microns.
Conventional pre-treatment is not considered in the design as performance of membrane
technology outpaces use of old methods of water treatment.
Figure -1 - 3D Perspective of a Typical Desalination Plant with Membrane Filters and RO
Modules
The sea water, termed as source water throughout this document, will be drawn through four
(4) off-shore intake pipes with velocity cap on each pipe. The quality of the source water is
critical for selection of pre-treatment equipment and to reduce foulants' loading on the RO
membranes. This design is based on the open intake system and submerged water inlet
structure. The design of velocity cap and sea water intake pipes must be carefully evaluated
to maintain sustained source water quality.
The source water will be collected in one large intake basin suitably located away from the
shore-line. The Intake structure is furnished with stationary bar screens and traveling water
4
screens to remove large debris, algae and other large materials. The Intake is an open
structure with six vertical turbine type pumps (Including two standby pumps) mounted on top
of the concrete basin. Each pump is sized for 25% of total flow i.e. 300,000 m3/day (12,500
m3/hr.). The desalination plant consists of two Trains. Each train receives 600,000 m3/day
(25,000 m3/hr.) of source water and consists of identical Pre-treatment equipment, RO unit
and Re-mineralization Unit.
A Gantry crane of capacity of 15 Ton main hook and 5 Ton capacity auxiliary hook are provided
to maintain pumps and traveling water screens. The design of intake basin and hydraulic study
of intake pumps and basin will be performed during detailed design phase. A sand screen will
be provided in the pump discharge pipe, if needed.
Passing through the pre-treatment and filtration process, filtered water is stored in the
dedicated Filter water tank for each Train. Three 50% low pressure pumps for each train
supply filtered water to high pressure pumps upstream of RO unit for further processing. For
large SWRO desalination plant as this, RO system will be two-stage process with an energy
recovery system (ERS). Refer to the Attachment 1 for two-stage RO and ERS operation.
Permeate from the first stage RO modules flow to the product tank. The brine concentrate
from the first stage RO flows to the Pelton wheel assembly. The hydraulic force turns Pelton
wheel, which is coupled with the high pressure pump motor shaft. The additional driving
energy applied to pump shaft reduces motor shaft power. The concentrate brine from Pelton
wheel enters into the second stage RO membrane. The permeate from second stage RO
module flows to the product tank and the brine will be discharged to the Brine outfall.
De-gasifier tower where dissolved gases are removed as RO membrane can’t stop gases.
Finally, permeate is stored in the Product tank. The product water is not yet good for drinking
as it needs to be disinfected and mineral needs to be added before it is ready to supply to the
water distribution network. Disinfectant chemicals such as sodium hypo-chloride or sodium
sulfate will be added in the product water discharge piping through static mixer.
MF and UF filters are to be backwashed on a regular basis. Waste from filters will be piped to
an outfall suitably permitted by the appropriate Authority having jurisdiction.
To be conservative, 50% recovery is used in the design. This means that approximately 50%
of the feed water will be rejected with saline concentration of approximately 70,000 ppm.
The brine reject has to be discharged considerable depth below the water level and
approximately at 1500 meter away from the shore-line. The requirement is that brine outfall
must be upstream of the intake point so that brine can be well mixed with the sea water.
5
3.0 Desalination Plant Design Bases:
3.1 Benefits of Membrane Technology:
This Table presents recommended filtration process based on particle sizes. The
particle separation are performed through (1) Particle Filtration, (2) Microfiltration,
(3) Ultrafiltration, (4) Nano filtration and (5) Reverse osmosis. Range of particle size as
shown above is in Angstrom and micrometer scale.
Angstrom scale: 1 to 107
Micrometer scale: 10-4 to 1000
Corresponding Micron (μ) conversion: 1 micrometer (μm) =1 micron (μ)
Therefore, the selection of filtration equipment can be chosen as below:
a) Particle Filtration: This falls under conventional water treatment employing media
filtration such as gravity filters and or pressure filters. The range of particle size
that it can handle particles larger than 100 micron
b) Microfiltration (MF): This applies to membrane filtration process. The particle size
that MF filtration can handle is 0.1 micron to 1 micron.
c) Ultrafiltration (UF): This applies to membrane filtration process. The particle size
that UF filtration can handle is 0.01 to 0.1 micron.
d) Nano filtration (NF): From performance standpoint, NF is almost equal to UF
membranes. So, either membrane filters can be used. This design proposes UF
filtration as it is widely used in the desalination industry.
6
e) Reverse Osmosis (RO): The RO membrane process is superior to all other
filtration methods. The particle range that RO membrane can separate up to
micron to 0.01 micron. The subject design includes MF, UF and RO membranes to
produce permeate water.
3.2 Source Water Constituents:
Source water (Feed Water) constituents are critical parameters in design and selection of pre-treatment equipment. The key factor of designing a desalination plant is to collect detailed water analysis and seasonal data with high, low and average numbers of the following constituents:
a) Silt density index (SDI): This parameter indicates the particulate fouling potential of the source water. Generally, RO supplier prefers a SDI level less than 5. Membrane filtration can bring the SDI down below 3. The design includes SDI 5 in source water.
b) Total suspended solids (TSS) and Turbidity: TSS is a measure of weight of the
particulates in the sea water expressed in mg/L unit. Turbidity is the haziness of
the fluid caused by particulates. It is indicative of clay, silt, suspended organic
matters and microscopic aquatic life. Turbidity is expressed as NTU
(Nephelometric turbidity unit). The ratio of TSS and Turbidity is important design
parameter for selection of pre-treatment equipment. The maximum TSS
considered in the design is 8 mg/L and turbidity 5. Therefore, TSS and Turbidity
ratio is 1.6. TSS could increase to a high level due to algal growth impacting
membrane performance. Therefore, location of the sea water intake is critical
where TSS and turbidity ratio falls within the specification limits of MF/UF filters.
Usually, turbidity of the filtered water below 0.1 NTU is desirable for RO
membranes.
c) Total Dissolved Solids (TDS): Refer to Attachment 2 for a typical Mediterranean Sea
and Red sea water analysis and expected TDS level downstream of the RO Unit.
This TDS level is an important water quality parameter in determining the feed
pressure at the inlet to the RO membranes. Every 100 mg/L of TDS in the source
water creates 0.07 bar (1 psi) osmotic pressure [1]. Therefore, 35,000 mg/L TDS
level in Mediterranean Sea water (Attachment 2), osmotic pressure is 24.5 bar
(350 psi). Based on this osmotic pressure and pressure losses through piping,
valves and RO Modules, HP feed pump discharge pressure is selected at 50 bar.
d) Total Organic carbon (TOC): Saline water contains naturally occurring or man-
made organic compounds and aquatic micro-organisms. Since micro-organisms
and most organic molecules are large in size, they are rejected by RO membranes.
However, small organisms may form cake layer that impact RO performance.
Typically, sea water contains TOC less than 0.2 mg/L. However, TOC level of 2 to
7
2.5 mg/L triggers accelerated bio-fouling of RO membranes. Therefore, historical
data of the organic constituents should be reviewed during site selection process.
e) Dissolved Gases: Sea water contains oxygen, hydrogen, and ammonia and
hydrogen sulfide gases. These gases pass through RO membranes. These gases are
to be degasified to make desalinated water suitable for drinking.
3.3 Product (Permeate) Recovery Factor:
Percent Recovery is the amount of water that is being recovered as permeate water.
The higher the recovery percentage means that less water is directed to drain as
concentrate and saving more permeate water. However, if the recovery percentage is
too high for the RO design then it can lead to larger problems due to scaling and
fouling. The percentage recovery for a RO system is established based on numerous
factors such as feed water chemistry and RO pre-treatment before the RO system. The
percentage recovery is the ratio of permeate flow to the feed flow:
% Recovery= 𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒 𝐹𝑙𝑜𝑤 𝑅𝑎𝑡𝑒
𝐹𝑒𝑒𝑑𝑤𝑎𝑡𝑒𝑟 𝐹𝑙𝑜𝑤 𝑅𝑎𝑡𝑒𝑥100
Large commercial RO system typically run anywhere from 50% to 85% recovery
depending on the feed water characteristics and other design consideration.
Recovery shown in this diagram is for one Train only. Other Train's recovery is identical.
8
3.4 Product (Permeate) Recovery Factor:
The concentration factor is related to the RO system recovery and is an important
consideration for RO system design. The more water recovered as permeate (the
higher the percentage recovered), the more concentrated salts and contaminants are
collected in the concentration rejected stream.
This can lead to higher potential for scaling on the surface of the RO membrane when
the concentration factor is too high for the system design and feed water composition.
Concentration Factor= 1
1−𝑅𝑒𝑐𝑜𝑣𝑒𝑟𝑦 %
3.5 Energy Recovery:
There are three types of energy recovery systems are used in the desalination
industry. These are:
• Pelton wheel
• Hydraulic Turbo-charger
• Isobaric Pressure Exchange
A large portion of energy is stored in the concentrate produced in the RO system. This
energy can be calculated as below:
ERmax -is the energy recovered from the RO system expressed as percentage of the
energy the RO system with the feed flow.
Pfeed - is the feed pressure applied to the RO modules (bar)
Pdrop - is the pressure drop in the RO modules (bar)
R - RO system recovery (%)
ERmax = [(Pfeed - Pdrop) x (1-R)]/ Pfeed = [(50-5) x (1-0.5)]/50 = 45%
This means that if energy recovery equipment is 100% efficient, then it could recover
45% of the energy introduced in the RO system.
Pelton Wheel:
9
Pelton wheel is the nineteenth century technology developed for hydro-electric
power generation. The subject design includes this method to recover the energy.
However, it should be further reviewed for other methods and is to be optimized.
Turbo-Charger:
The Turbo-charger consists of a turbine and centrifugal pump on the same shaft. It is
also called Hydraulic Turbo-booster (HTB). The pump boosts up the feed pressure to
the RO system. The HTB system improves pumping efficiency 80 to 85%. The
concentrate from the HTB will go to the second stage RO module.
Isobaric Pressure Exchange:
Energy recovery system working on the pressure exchange system delivers energy in
the concentrate directly via piston and deliver new source water into the RO system.
Flowserve Company developed Dual Work Exchanger Energy Recovery (DWEER)
system that claims 98% energy is recovered from the brine stream. Therefore,
Flowserve unique design should be reviewed while finalizing which energy recovery
method to be used in the subject project.
10
4.0 Intake Water System:
The function of the water supply intake is to extract and deliver the water to the users
such as power plants, municipal and other users. For desalination plants in coastal
areas, the main source is the sea water, which is subject of this section.
The most important element in the design of an intake to extract water from a water
body is selecting its location and configuration. Problems encountered in the operation
of some intakes is improper selection of the intake location or improper design and
construction to fit the site condition. Several factors affect such conditions: owner
preference for the site, lack of adequate baseline site date, economics, constructability,
and failure to provide documents to the engineer to address the siting criteria. Based
on field experience, the following is a summary of factors considered in citing and
locating a water intake:
a) Water availability and dependability
b) Water quality
c) Bathymetry of the sea in coastal and effect on water depth
d) Sediment transport and drift in coastal areas
e) Aquatic life protection
f) Wave condition in coastal areas
g) Intake hydraulics
h) Constructability
i) Initial and maintenance dredging including disposal of the dredged material
j) Operation and maintenance of the intake system.
4.1 Selection and Description of Intake Water System
1. Based on the above elements and past experience in locating and
designing water intakes along the Mediterranean Sea Coast, an offshore
intake system is proposed for the desalination plant.
2. An offshore intake consists of a vertical shaft protruding above the sea
bed and connected to a horizontal velocity cap located below the
minimum sea water level. The velocity cap and its vertical shaft are
connected to the pipe that conveys the water to an onshore pump intake
structure and related equipment. The velocity cap is provided to create a
horizontal velocity field to enable the fish to avoid entrapment by the
flowing water. It also includes trash bars to prevent the withdrawal of
large floating objects and intruders from entering the intake water
system.
11
3. Based on the above elements and past experience in locating and
designing water intakes along the Mediterranean Sea Coast, an offshore
intake system is proposed for the desalination plant.
4. An offshore intake consists of a vertical shaft protruding above the sea
bed and connected to a horizontal velocity cap located below the
minimum sea water level. The velocity cap and its vertical shaft are
connected to the pipe that conveys the water to an onshore pump intake
structure and related equipment. The velocity cap is provided to create a
horizontal velocity field to enable the fish to avoid entrapment by the
flowing water. It also includes trash bars to prevent the withdrawal of
large floating objects and intruders from entering the cooling water
system.
5. Control biofouling in the offshore velocity cap and the water conveying
pipes, Sodium Hypochlorite, which is a widely used chemical, is injected
inside the velocity cap though a multiport diffuser.
6. Minimizing the withdrawal of sea bottom by the flowing water is an
essential element in locating offshore velocity cap above the sea bed.
Hydrographic studies in coastal areas have shown that in a depth typically
10 m below sea water level littoral drift diminishes. This depth may be
achieved at 1000 M offshore in the Egyptian coast of the Mediterranean
Sea and shall be considered for the location of the velocity cap based on
the associated bathymetric survey.
7. The total flow required for the proposed facilities is 1,200,000 m3/day, the
equivalent of approximately 14 m3 /sec. To facilitate construction and to
provide redundancy during operation, four offshore intakes velocity caps
are proposed. Each is connected to an offshore pipe to convey the water
to the onshore common pump intake structure.
8. The velocity caps will be identified by dolphins or floating buoys as a
warning to boaters or navigators.
4.2 Water Conveying Pipes
1. The proposed pipe is HDPE with a nominal diameter of 2000 mm (2m). For
a discharge of 3.50 m3 /sec, the flow velocity in each pipe will be
approximately 1.50 m/sec. This type of pipe is mostly used in desalination
plants because of the low flow rate and ease of installation as compared
to concrete pipes or GRP, which require trenching and protection by
12
riprap and filter layers. This type of protection is needed to prevent
movement of the pipes by wave induced forces and currents, and by
ambient sea currents. The HDPE pipe will be loaded with anchor blocks to
protect the pipe from flotation due to buoyance and wave induced uplift
and drag forces. At the entrance to the onshore pumping station, each
pipe will be provided with a gate or a suitable valve for isolation.
2. The offshore pipes when reaching shallow water approximately (- 3 to - 4
m) below seawater level, they will be installed in dredged trenches or a
combined trench. The pipes will be covered by filter layers and riprap to
resist erosion and to prevent the pipe from the floatation. The completed
sea bed in the shallow area shall be formed to match the local sea bed.
3. Between the shoreline and the pump intake structure, the pipes will be
installed in a trench which is a continuation of the near shore trench. The
top of the pipe must be below the hydraulic grade line to maintain gravity
flow to the intake structure.
4.3 Pump Intake Structure
The pump intake structure should be located onshore and far inland from the
maximum tide water level and the associated wave run-up. The location also should
comply with local building regulations.
The function of this structure is to deliver the water to the desalination equipment. The
structure includes:
1. Water filtration system: Trash racks and traveling water screens
2. Settling basin to allow for the deposition of coarse sea sediment that
may be carried by the flow during adverse sea conditions. The sediment
should be dredged as needed during plant operation.
3. Piers
4. Stop log slots
5. Pumps of various capacities and uses
6. Chemical injection manifold
7. Curtain walls to improve the flow conditions approaching the pumps
8. Baffle walls, filets, and splitters
9. Access hatches and ladders.
5.0 Brine Water Discharge:
The brine water discharged from the processing facilities contains highly concentrated
chemicals. To minimize the impact on the aquatic habitats and recirculation into the offshore
intakes, the water must be discharged in an acceptable method. This discharge is
accomplished by the use of a multiport diffuser located near the sea bed and away from the
velocity caps.
13
The approximate total estimated brine discharge flow rate is 500,000 m3 / day. The diameter
of the discharge pipe could be similar to the offshore intake pipes. The diffuser ports are to
be determined from numerical thermal model to achieve an acceptable configuration and
location. The location would be approximately 1500 m offshore to minimize recirculation.
The installation of the brine pipe in the near shore, shall be similar in concept to that of the
offshore the intake pipes. However, the onshore pipe between the plant and the shoreline
shall be buried, but its grade shall meet the hydraulic design requirements of the discharge
system.
6.0 Pre-treatment of Sea Water:
Pre-treatment system consists of Cartridge filter assembly and combination of micro-filters
and ultra-filters as detailed below:
a) Cartridge filters - Cartridge filters can remove particles of sizes 1 through 25
micron. For the subject design, 25 micron size cartridge filters upstream of the
MF/UF membranes is a conservative selection. Based on the loading rate of 5 gpm
per 40" length and 8" element diameter, 5000 cartridge filters (each filter 40 inch
long) are required for each Train. Therefore, if TSS level is very low in the sea water
analysis and good screening of sand and marine aquatic are ensured in the intake
traveling water screen, cartridge filters can be fully eliminated. The associated cost
vs. the associated benefit must be evaluated during detailed design stage.
Alternatively, micro-strainer on each intake pump discharge pipe could be
installed at much a lesser cost, if needed.
b) Combination of micro-filter (MF) and Ultra-filter (UF)
Micro-filtration removes particles of 0.1 to 1 micron size. In general, TSS and large
colloids are rejected while micro-molecules and dissolved gases will pass through.
It removes bacteria and some viruses.
Ultra-filtration provides macro molecular separation of micron size up to 0.1
micron. MF and UF filters are available in various diameters and lengths. The
subject design includes 8" dia and 40" long MF and UF filter elements.
c) Quantity and size
MF and UF membrane systems generally use hollow fibers that can be operated
as outside-in or inside-out direction of flow. Refer to Attachment 3 that includes
design basis and quantity of filters required per Train. MF/UF filters are arranged
in horizontal configuration with piping, valve and controls as presented below:
14
7.0 Desalination RO System:
The high pressure pump continuously feed filter water into the RO membrane. Feed water is
split into (1) low saline called permeate or product water and (2) high saline called brine
concentrate. Permeate flux and salt rejection are key performance parameters for the RO
system.
Permeate flux is given by the RO supplier.
Salt rejection is calculated as below:
TDSfeed = salt level in the feed water, 35,000 mg/l
TDSconcentrate = salt level in the brine reject
TDSpermeate = salt level in permeate, assume 200 mg/l
R= recovery =50%
TDSconcentrate = 𝑇𝐷𝑆𝑓𝑒𝑒𝑑 − 𝑇𝐷𝑆𝑝𝑒𝑟𝑚𝑒𝑎𝑡𝑒 𝑥50%
100
𝑅
100
=35,000−200𝑥0.5
0.5= 34,900/0.5 = 69,800 mg/l
Flux and rejection are intrinsic properties of the membrane performance. These two are
influenced by variable parameters like temperature, pressure, salt concentration in the feed
water and recovery. Final design must take in consideration into these influence factors for
best RO performance and cost optimization.
8.0 Degasification and Re-mineralization:
8.1 Degasification
RO membranes pass dissolved gases. These are oxygen, carbon dioxide, hydrogen
sulfide and ammonia. Typically, Hydrogen sulfide is not present in sea water. All gases
will be released through the degasifier tower as shown in Attachment 1.
8.2 Re-mineralization
Product water from Desalination plant is low on mineral content, hardness, alkalinity,
and ph. Therefore desalinated water must be conditioned prior to discharge to the
Municipal distribution network. This post-treatment of the desalinated water is the
re-mineralization process. It involves (1) mineral addition to protect public health (2)
prevention of corrosion in downstream piping system, (3) disinfection to maintain
biological stability and (4) removal of boron, silica, and gases that cause taste and
odor.
a) Calcium addition: calcium hydroxide (hydrated lime) and carbon dioxide is
added to provide hardness and alkalinity.
15
2CO2 + Ca (OH) 2 Ca (HCO3)2
Calcium can be stored on site as powdered hydrated lime or as calcium oxide (pebbled
lime). Carbon dioxide is delivered in liquefied form and stored under pressure in steel
tank. The hardness level in the permeate water is 80 - 120 mg/l as CaCO3. Sulfuric acid
or carbonic acid is added to maintain pH and lime solubility. The re-mineralization is
an important process to make the desalinated water good for drinking and will be
further reviewed during detailed design phase.
Sodium hypochloride is added to disinfect water from bacteria. Sometime ultra-violet
(UV) injection could be necessary.
Based on worldwide experience and economics, water quality after re-mineralization
should be maintained at:
a) Alkalinity >80 mg/l as CaCO3
b) 80 <Ca2+ <120 mg/l as CaCO3
c) 3<CCPP<10 mg/l as CaCO3
d) 7.5 <pH <8.5
e) Larson Ratio <5
8.3 Plant Equipment Sizing and Associated Auxiliary Load
The plant equipment sizing for the proposed 800,000 meter cube per day SWRO plant
is exhibited in Attachment 3. The preliminary plant auxiliary load for the proposed
plant is shown in Attachment 4.
9.0 Conclusion:
The Desalination plant of 800,000 m3/day will be largest plant in today's desalination market.
Therefore, the design is critical and considerable time need to be allocated for design
evolution.
Site selection is important for this size desalination plant. A good quality sea water would
provide substantial capital cost reduction. This project has two identical Trains of capacity
400,000 m3/day each. Site should be selected for additional Trains at a later date. This would
reduce cost of Intake system for the expansion units. The site selection criteria are outlined
in Attachment 5.
16
ATTACHMENT 1
17
ATTACHMENT 2
This attachment is based on data taken from the Desalination Engineering
Planning and Design Manual by Voutch Nokolay – Water Use Association
Table 1: presents typical Red sea water analysis, permeate water analysis and final
product water analysis following remineralization process.
Table 1
38,000
18
Table 2 presents typical Mediterrenean sea water analysis, permeate water analysis and final
product water analysis following remineralization process.
Table 2
35,000
19
Table 3 presents large RO membranes elements currently available in the market. RO element
of 16 and 18 inch diameter and each 40 inch long is the latest RO elements used in the
desalination plant. This large size RO elements is a remarkable progress in the RO design that
increased permeate flow rate and reduced quantity of RO elements as compared to 8 inch
dia x 40 inch long RO elements used a decade ago.
RO element of 16" dia x 40" long have been considered for the subject project.
Table 3
20
ATTACHMENT 3
Equipment Sizing Calculations:
Equipment Units Calculations Results
PUMPS
Intake Pump
Intake Pump, Flow m3/day 800,000 + 50%(800,000)= 1200,000
1,200,000
Per Pump m3/day 1200000
4=300,000 300,000
Per Pump m3/hr. 300,000
4
12,500
Per Pump lpm 12500𝑥1000/60 208,333
Per Pump gpm 208,333/3.8=54,824 Say 55,000
Pump Head bar 2 bar pipe loss +2 bar Catridge filter loss + 2 bar MF/UF Filter loss +2 bar Filter tank static head loss
8
Brine Pump
Brine Discharge Flow m3/day 144,000 Rejected brine from RO + 60,000 backwash
204,000
Brine Discharge Flow m3/hr 204,000
24
8,500
Brine Discharge Flow gpm 8,500𝑥264
60
37,400
No. of operating Pump 3 x 50% Pumps 2 operating
Flow per Pump gpm 37,400
2
18,700 Say 20,000
Pipe dia mm 1400
Pump Head bar 4
Filter Pump
No. Filter Pump 3 x 50%
No. Filter Pump Operating
2
Flow gpm 20,000
2
10,000
Head bar 3 bar at MF/UF +2 bar piping & valve loss
5
HP Feed Pumps 3 per Train 2 operating
Flow m3/day 540,000
Flow m3/hr 540,000
24
22,500
21
Flow/Pump m3/hr 22,500
2
11,250
Flow/Pump gpm 11,250𝑥264
60
49,500 Say 50,000
Head bar 30 bar at RO +5 bar piping & valve loss +10 bar loss in Pelton wheel + 5 bar margin
50
Product Pumps 4 per Train 3 operating/Train
Flow m3/day 396,000
Flow/Pump m3/hr 396,000
3𝑥24
5,500
Flow/Pump gpm 5,500𝑥264
60
24,200
Head bar 5 bar Loss in Remenaralizer+10 bar at supply network
15
Catridge Filter
Length inch 40
Flow flux 5 gpm/10 inch
20
Flow per tube gpm 40𝑥5
10
20
Total Flow/Train m3/day 540,000
Total Flow/Train m3/hr 540,000
24
22,500
Total Flow/Train gpm 22,500𝑥264
60
99,000
Qty of Catridge Filter 99000
20
4,950 Say 5000/Train
MF/UF Filters
Length inch 40
Flow flux 5 gpm/10 inch
20
Total Flow/Train m3/day 540,000
Design Flux Lmh 56.5
Total membrane Area required
m2 540,000𝑥1000
56.5𝑥24
398230
Area/Module m2 40
NO. of module 398230
40
9955 Say 10,000/Train
22
No.Vessels 10,000
8
1250
RO Modules
Estimate with 16" dia & 40" length
10,000
2
5000 tubes/Train
Vessel (8 tube in 1 vessel)
5000
2
2,500 vessels/Train
Tanks
Filter water tank m3 1 hour storage, capacity 12,500
Dia x height m 33 x 15
Product Tank m3 1 hour storage, capacity 16,667
Dia x height m 34 x 18
Brine Storage Tank 1 hour storage, capacity 8,500
Dia x height m 30 x 12
23
ATTACHMENT 4 PLANT AUXILIARY LOAD
Description Total Qty Operating
Flow gpm
Head bar
Head ft BHP KW/eqpt Train
Motor KW
Total Load MW Remark
Intake
Intake Pump 6 4 55000 8 262.4 4373 6514 26,055 Continuous
Traveling water screen 4 4 Continuous
Screen Wash Pump 3 2 500 15 492 75 111 222 continuous
Crane 1 1 Intermittent
Chemical injection pumps 10
Brine Discharge
Brine pump 3 2 20000 6 196.8 1193 989 1,977 continuous
Filter Water Pump/Train 3 2 10000 5 164 497 412 2 1,648 continuous
HP Feed Pump/Train 3 2 50000 50 1640 24848 20597 2 82,387 continuous
Product Water Pump 3 2 24200 15 492 3608 2991 2 11,963 continuous
Degasifier Pump 3 2 24200 4 131.2 962 798 2 3,190 continuous
Remineralization 80 2 160 continuous
Total 127,611
Misc Electrical Load (30%) 165,894
Margin 20% 166
199,073
Station Load, MW 199
Rounding Station Load, MW 200
24
ATTACHMENT 5
Field and Analytical Studies for Siting and Designing
Desalination Plant and
Its Related Offshore Structures
A. Site Selection Studies
To select a coastal site for power plants or desalination plants requires a comprehensive
evaluation of several widely used parameters. This evaluation will be used as a tool in
selecting a favorable site from at least two potential sites.
The site selection includes a search for available data, performance of field surveys, and
on-site observations as discussed below:
• Ease of access to the site from the land and the sea
• Space availability to accommodate the proposed facilities and laydown during
construction with an estimated area of two square kilometers
• Site grade with respect to high seawater level
• Presence of transmission lines and location of a nearby existing power plant able of
supplying power for desalination
• Geotechnical information on offshore conditions. This may be obtained from
investigations made for existing power plants, nearby piers or jetties, and other sea
front structures.
• Bathymetric survey of the sea in front of the site that extends to the high tide line and
covers an area matching the width of the site at the shoreline as a minimum. The
offshore survey should extend at least 1600 m measured from the low tide level. The
onshore segment of the survey should extend beyond the high tide level.
• Historic sea water tide conditions and water quality. If the potential sites are in the
same region of or at an existing power plant, the water quality is not critical for siting
but is needed for design and equipment selection. This data can be collected from
samples obtained during the bathometric survey. The survey and water quality
sampling shall be made only after identifying the most suitable sites or site.
• Sea bottom sedimentation type and gradation
• Aquatic life type and environmental protections
• Presence of seaweed and floating trash. This should be identified at any site from field
observations and experience gained from existing power plants or other marine
installations.
• Other parameters as may be determined from field observations by specialist
25
B. Field and Analytical Studies and Design
The required data collection programs and studies for the design and construction of the
water supply for the desalination plant depend on the site location, hydrographic and
geotechnical onshore and offshore conditions. The selected contractor shall determine
the applicable studies required for the given site. The following is a list of typical studies
that are required at a given site for: licensing, design, and construction of a desalination
plant. Additional studies also may be required depending on the site conditions and
desalination plant requirements:
• Hydrographic data collection
• Historic tide levels
• Bathymetric and hydrographic surveys
• Historic site and /or regional wave climatic data
• Topographic surveys and subsurface investigations
• Site topographic surveys
• Offshore subsurface investigations
• Onshore subsurface investigation and geologic assessment
• Hydraulic analysis and design
• Seawater level and wind studies
• Determination of maximum and minimum water levels
• Plant grade and shore protection
• Brine discharge numerical modeling
• Wave force analysis and design of the offshore velocity caps and the brine outfall
diffuser
• Wave uplift forces on the unburied pipe in the deep offshore water, including offshore
pipes stability analysis to overcome uplift forces and buoyancy and design protection
• Riprap sourcing and laboratory testing
• Shore protection and trenching and protection of the intake pipes and the brine pipe
near shore and offshore
• Pump intake physical hydraulic model study
• Pump performance test
• Other tests and studies.